2004 was the year in which Berryz Koubou and Hello Pro Egg were formed, Abe Natsumi, Tsuji Nozomi and Kago Ai graduated from Morning Musume, Mika Todd graduated from Coconuts Musume and Hello! Project, V-u-den and W were formed.
- January 3: W is formed.
- January 14: Berryz Koubou is formed
- January 25: Abe Natsumi graduates from Morning Musume.
- May 2:
- June: Hello Pro Kenshuusei is formed.
- August 1: Tsuji Nozomi and Kago Ai graduate from Morning Musume.
- August 10: V-u-den is formed.
- January 19: Ise Layla was born.
- February 9: Tamenaga Shion was born.
- February 22: Ishiyama Sakura was born.
- February 28: Okamoto Honoka was born.
- March 16: Kitagawa Rio was born.
- April 19: Sato Hikari was born.
- April 20: Ishiguri Kanami was born.
- April 23: Kawano Minori was born.
- April 30: Yonemura Kirara is born.
- June 29: Kitagawa Ryo was born.
- July 23: Kubota Nanami was born.
- September 1: Doi Rena was born.
- September 28: Kudo Yume was born.
- October 18: Fukuda Marin was born.
- October 20: Okamura Minami was born.
- November 1: Hashimoto Ririka was born.
- November 10: Kusunoki Mei was born.
- December 22: Kiyono Momohime was born.
- January 1: Sarasara no Kawa - Maeda Yuki
- January 21: Ai Araba IT'S ALL RIGHT - Morning Musume
- January 28: Kiseki no Kaori Dance. - Matsuura Aya
- February 4: Aegekai ni Dakarete - Iida Kaori (debut)
- February 11: Genki no Nai Hi no Komoriuta / Nagaragawa no Hare - Nakazawa Yuko
- February 25:
- March 3: Anata Nashide wa Ikite Yukenai - Berryz Koubou (debut)
- March 10: Hyacinth - Matsuura Aya
- March 17: Sayonara no LOVE SONG - Goto Maki
- April 21: Lucky Cha Cha Cha! - Minimoni
- April 28: Fighting Pose wa Date ja nai! - Berryz Koubou
- May 12: Roman ~MY DEAR BOY~ - Morning Musume
- May 19: Koi no Vacance - W (debut)
- May 26:
- June 2: Datte Ikitekanakucha - Abe Natsumi
- June 9: Namida no Taiyou - Melon Kinenbi
- July 7: Yokohama Shinkirou - Goto Maki
- July 14: YOUR SONG ~Seishun Sensei~ - Matsuura Aya
- July 22: Joshi Kashimashi Monogatari - Morning Musume
- July 28: Door no Mukou de Bell ga Natteta - Iida Kaori
- August 4: Shining Itoshiki Anata - Country Musume ni Konno to Fujimoto
- August 11: Koi no Telephone GOAL - Abe Natsumi
- August 18: Aa Ii na! - W
- August 25: Happiness ~Koufuku Kangei!~ - Berryz Koubou
- September 23: Koi no Nukegara - v-u-den (debut)
- September 29: Nishi Shinjuku de Atta Hito - Maeda Yuki
- October 6: Ren'ai Sentai Shitsu Ranger - Nochiura Natsumi
- October 14: Robo Kiss - W
- October 20: Watarasebashi - Matsuura Aya
- October 27: Champagne no Koi - Melon Kinenbi
- November 3: Namida ga Tomaranai Houkago - Morning Musume
- November 10: Koi no Jubaku - Berryz Koubou
- November 17: Sayonara "Tomodachi ni wa Naritakunai no" - Goto Maki
- November 26: Tensai! LET'S GO Ayayamu - Matsuura Aya and Ecomoni
- December 1: ALL FOR ONE & ONE FOR ALL! - H.P. All Stars
- January 1: ×3 - Matsuura Aya
- January 28: 2 Paint It Gold - Goto Maki
- February 4: Hitoribocchi - Abe Natsumi
- February 11: Minimoni Songs 2 - Minimoni
- March 3: JECICA - Michiyo
- March 31: Best! Morning Musume 2 - Morning Musume
- June 2: Duo U&U - W
- July 7: 1st Chou Berryz - Berryz Koubou (debut)
- July 22: Dai Nishou ~Tsuyogari~ - Nakazawa Yuko
- December 1: THE Nimaime - Melon Kinenbi
- December 8: Ai no Dai 6 Kan - Morning Musume
- December 15: Morning Musume EARLY SINGLE BOX - Morning Musume
- December 22: Petit Best 5 - Hello! Project
- December 29: Avenir ~Mirai~ - Iida Kaori
- April 3: Morning Musume CONCERT TOUR 2004 Haru The BEST of Japan
- May 28: Morning Musume DVD MAGAZINE Vol.1
- June 19: Morning Musume no Kiseni no Hoshi • Chikyuu ~Necchii Chikyuu wo Samasunda.~
- July 31: Morning Musume Tsuji☆Kago Sotsugyou Memorial
- August 21: Morning Musume DVD MAGAZINE Vol.2
- January 28: Morning Musume's major debut 6th anniversary
- September 14: Morning Musume's formation 7th anniversary
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